1. Field of the Invention
The present invention relates to an optical detecting sensor and, more particularly, to a thin film transistor (TFT) type optical detecting sensor.
2. Description of the Related Art
Generally, optical detecting sensors are used in facsimile and digital copying machines, and in fingerprint recognition systems as an image reader. In recent years, a TFT type optical detecting sensor has been suggested. The TFT changes its electrical characteristics in response to incident light. A TFT type optical detecting sensor is a system using such a TFT having such a characteristic.
FIG. 1 shows a plan view of a conventional TFT type optical detecting sensor and FIG. 2 shows a sectional view taken along line II--II of FIG. 1.
As shown in the drawings, an optical detecting sensor 100 comprises a window 8 through which light generated from a light source 102 passes and a sensor TFT 6 for generating optical current by detecting the light which is transmitted through the window 8 and then reflected from an object 12.
Since the optical detecting sensor 100 is designed to detect the light passing through the window and reflected from the object 12, it is essential that the window 8 has a sufficient light passing area. In addition, since a storage capacitor 4 for storing charges generated by the reflected light has to maintain a predetermined capacity, it is also essential to provide a sufficient storage area to the storage capacitor 4.
As shown in FIG. 1, a pixel of the optical detecting sensor 100 is comprised of a storage capacitor 4 and a switching TFT 2 in addition to the window 8 and the sensor TFT 6. Generally, an area of one pixel defined by A.sub.1 *(B.sub.1 +2B.sub.1 '), where A.sub.1 is defined by a sum of C.sub.1, D.sub.1, E.sub.1, and F.sub.1. Accordingly, an area of the window 8 can be defined by (B.sub.1 +2B.sub.1 ')*F.sub.1, and an area of the storage capacitor 4 can be defined by (B.sub.1 +2B.sub.1 ')*D.sub.1.
The optical detecting sensor 100 will be described more in detail with reference to FIG. 2.
In FIG. 2, the switching TFT, the storage capacitor, the sensor TFT, and window are defined by regions 2, 4, 6 and 8, respectively.
A first metal layer is formed on a substrate 1. The first metal layer comprises a gate electrode 20 of the switching TFT 2, a first storage electrode 30 of the storage capacitor 4, and a gate electrode of the sensor TFT6. The first metal layer is made of a material selected from the group consisting of W, Mo, Cr and Al.
In addition, a gate insulating layer 14 is disposed on the first metal layer, and a semiconductor layer is deposited on the gate insulating layer 14. The semiconductor layer is patterned such that semiconductor elements 26 and 46 are formed to act as an active layer of the switching TFT 2 and the sensor TFT 6, respectively.
A second metal layer is deposited and patterned to form drain electrode 24 and source electrode 22 on the active layer of the switching TFT 2, a second storage electrode 34 for the storage capacitor 4, and drain electrode 44 and source electrode 42 on the active layer of the sensor TFT 6.
An insulating layer 16 is formed to protect the switching TFT 2, the storage capacitor 4 and the sensor TFT 6. A light interrupting layer 18 is formed on a portion of the protecting insulating layer 16 corresponding to the switching TFT 2 to block light scattered from the object 12, and a protecting layer 10 is deposited on the insulating layer 16 and covers the light interrupting layer 18.
Generally, an active layer 46 of the sensor TFT 6 is made of a-Si:H which has a low dark conductivity and a high optical conductivity.
Since the sensor TFT 6 is operated by optical current in accordance with the intensity of incident light in an off-state, negative voltage is always applied to the gate electrode 40 to maintain the off-state. An optical current is generated in proportion to the intensity of the incident light, and is directed to the second storage electrode 34 of the storage capacitor 4 through the source electrode 42, and then stored in the storage capacitor 4 as charges.
Furthermore, when a bias voltage is applied to the gate electrode 20 of the switching TFT, the charges stored in the storage capacitor 4 are conducted to the source electrode 22 through the drain electrode 24 of the switching TFT 2.
In the above-described sensor, in order to receive more light, the area of the window 8 should be maximized.
It is also necessary to increase the capacity of the storage capacitor in order to store more charges.